<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 200px;
      height: 200px;
      background-color: red;
     
    }
    div:hover {
      background-color: blue;
    }

    /* 嵌套  & 表示当前元素 */
    /* div {
      width: 200px;
      height: 200px;
      background-color: red;
      &:hover {
        background-color: blue;
      }
    } */
    
  </style>
</head>
<body>
  <div></div>
</body>
</html>